1. Field of the Invention
The present invention relates to a thermosensitive recording material having excellent thermal response and giving a small amount of a deposit on a thermal head.
2. Related Art
Thermosensitive recording materials generally comprise a support and a thermosensitive recording layer provided thereon and containing, as major components, an ordinarily colorless or slightly colored dye precursor and an electron accepting developer. When heated by a thermal head, a thermal pen, a laser beam or the like, the dye precursor instantaneously reacts with the developer to form a recorded image. Thermosensitive recording materials are disclosed in Japanese Patent Examined Publication Nos. 43-4160, 45-14039, etc. Such thermosensitive recording materials have such advantages as recording is made with relatively simple equipment, maintenance is easy and no noise is generated during recording; therefore, said recording materials are in wide use in recorders for measurement, facsimiles, printers, terminal devices for computers, labels, automatic vending machines for railroad tickets, etc. Particularly in the field of facsimiles, demand for facsimiles of thermosensitive mode has greatly increased and, in parallel therewith, it has been pushed forward to develop facsimiles of higher speed in order to reduce the cost for signal transmission and/or facsimiles of lower energy consumption by making them smaller in order to making them less expensive. Correspondently to this trend of facsimiles to higher speed and lower energy consumption, higher sensitivity has been demanded for thermosensitive recording materials. Meanwhile, the dot density of thermal head was generally 8 lines/mm but has recently become higher (e.g. 16 lines/mm) and, in addition, the area of each dot has become smaller. Hence, it has been required to print small-sized characters in higher image quality or to print characters with density gradation by Dither method. Thus, better printability, namely, to form images having faithfully reproduced head dots has been required more than before.
When, in order to satisfy the above requirements, adhesion between a recording sheet and a thermal head is improved by subjecting the recording sheet to supercalendering of stronger degree, there appear defects such as decreased whiteness (so-called, background stain) and the like.
It is proposed in Japanese Patent Application KOKAI (Laid-Open) No. 56-27394 to provide an undercoat layer between a thermosensitive recording layer and a base paper. By the provision of such an undercoat layer, it has become possible to obtain an image of higher density with a small energy without applying strong supercalendering and thereby to achieve higher sensitivity. It is believed that the provision of this undercoat layer would be effective to smoothen the unevenness of the surface of a support and thereby to making smooth the surface of a thermosensitive recording layer.
As described above, by the provision of an undercoat layer, recording of higher sensitivity became possible. However, requirements in recent years for recording of still higher sensitivity and more improvement in dot reproducibility cannot be satified only by the provision of an undercoat layer which simply aims at smoothening the surface of thermosensitive recording material.